From your description it sounds as if you’re dealing with a solid residue left behind by the evaporation of a liquid, and it sounds as if you suspect that the stuff is (or was originally) of a pharmaceutical nature. Is the bottle sealed? Can it be opened to extract a sample of the white stuff? If some can be extracted you might persuade someone at a nearly university to run an x-ray spectrographic anaysis.
Because the chemical/pharmaceutical universe includes a great many white substances ranging in nature from inert to harmless to illegal and even to deadly, I doubt that you’d find any pharmacist, chemist or other potential authority willing to offer more than a very informal opinion (e.g., “could be residue from milk of magnesia — or not”). But they would be going out on a limb if their guess turned out to be seriously or even tragically wrong.